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Los cangrejos del género Callinectes están ampliamente distribuidos en aguas cálidas. Habitan principalmente en lagunas costeras y estuarios. En las lagunas del golfo de México, están sujetos a una pesquería importante. En el caso de México, Callinectes sapidus y Callinectes rathbunae contribuyen con 90 % de la captura nacional. Un aspecto importante en el ciclo de vida de los cangrejos es el parasitismo causado por balanos parásitos del orden Rhizocephala, debido a su alta prevalencia y periodicidad. El objetivo del presente estudio fue proveer de información reciente acerca del número de externas por individuo en cangrejos de la especie C. rathbunae. En un muestreo comercial realizado en marzo de 2019, se capturaron tres especímenes de la especie C. rathbunae parasitados con el balano rizocéfalo Loxothylacus texanus: un ejemplar macho con dos externas vírgenes, una hembra con cinco externas vírgenes, y un organismo macho con cinco externas maduras. Este es el primer reporte en donde se presenta evidencia de cinco externas en un hospedero, se concluyó que esta prevalencia fue baja. Es importante realizar más estudios de este tipo para comprender del todo el ciclo de vida de L. texanus y su impacto en las poblaciones de sus hospederos.

The system formed by a still-unidentified rhizocephalan infecting the Patagonian stone crab Danielethus (Platyxanthus) patagonicus (A. Milne-Edwards, 1879) was analyzed in northern Patagonia. Out of 3222 crabs sampled, mean prevalence of externae was 2.1%, while corrected mean prevalence based on observations of externae, scars or other indicators of infection was slightly higher (3.01%; N = 2100). Prevalence was higher in males (4.47%) than in females (1.44%). Parasitized males were morphologically feminized, while females showed no hyper-feminization. Although most parasitized crabs showed only 1 externa, 2 externae were observed in some individuals. The parasite externae were only present in intermediate-sized crabs (26.6-99.7 cm carapace width). While scanning electron microscopy images allowed detection of the ‘smooth-surface-balloon’ type of retinacula on the inner surface of the externae, typical of the Sacculinidae and Peltogastridae, the position of the mantle opening relative to the stalk, the receptacle location and the shape of the externae suggest that the parasite belongs to either the genus Sacculina or to the recently erected Parasacculina (Polyascidae).

Abstract This review is devoted to the pelagic larvae of rhizocephalan crustaceans with a kentrogonid type of development (Thecostraca: Cirripedia: Rhizocephala). These barnacles have undergone such a significant reduction of adult stages connected with the parasitic lifestyle that only the presence of a nauplius larva in their development indicates that this unique group belongs to crustaceans. The main attention is paid to the morphological characters of the larvae that can be used in the taxonomy and phylogeny of Rhizocephala. The differences between the larvae of peltogastrid and sacculinid types are discussed. The sexual dimorphism of cyprids associated with different functions of male and female larvae is analyzed.

The endoparasitic crustacean Sacculina carcini (Cirripedia: Rhizocephala) has a much simpler morphology than conventional filter-feeding barnacles, reflecting its parasitic lifestyle. To investigate the molecular basis of its refined developmental program, we produced a draft genome sequence for comparison with the genomes of nonparasitic barnacles and characterized the transcriptomes of internal and external tissues. The comparison of clusters of orthologous genes revealed the depletion of multiple gene families but also several unanticipated expansions compared to non-parasitic crustaceans. Transcriptomic analyses comparing interna and externa tissues revealed an unexpected variation of gene expression between rootlets sampled around host midgut and thoracic ganglia. Genes associated with lipid uptake were strongly expressed by the internal tissues. We identified candidate genes probably involved in host manipulation (suppression of ecdysis and gonad development) including those encoding crustacean neurohormones and the juvenile hormone binding protein. The evolution of Rhizocephala therefore appears to have involved a rapid turnover of genes (losses and expansions) as well as the fine tuning of gene expression.

By shuffling biogeographical distributions, biological invasions can both disrupt long-standing associations between hosts and parasites and establish new ones. This creates natural experiments with which to study the ecology and evolution of host-parasite interactions. In estuaries of the Gulf of Mexico, the white-fingered mud crab (Rhithropanopeus harrisii) is infected by a native parasitic barnacle, Loxothylacus panopaei (Rhizocephala), which manipulates host physiology and behaviour. In the 1960s, L. panopaei was introduced to the Chesapeake Bay and has since expanded along the southeastern Atlantic coast, while host populations in the northeast have so far been spared. We use this system to test the host's transcriptomic response to parasitic infection and investigate how this response varies with the parasite's invasion history, comparing populations representing (i) long-term sympatry between host and parasite, (ii) new associations where the parasite has invaded during the last 60 years and (iii) naïve hosts without prior exposure. A comparison of parasitized and control crabs revealed a core response, with widespread downregulation of transcripts involved in immunity and moulting. The transcriptional response differed between hosts from the parasite's native range and where it is absent, consistent with previous observations of increased susceptibility in populations lacking exposure to the parasite. Crabs from the parasite's introduced range, where prevalence is highest, displayed the most dissimilar response, possibly reflecting immune priming. These results provide molecular evidence for parasitic manipulation of host phenotype and the role of gene regulation in mediating host-parasite interactions.

The externae of parasitic sacculind, Sacculina leptodiae (Sacculindae: Rhizocephala: Cirripedia) on the xanthid crab, Leptodius exaratus (Xanthidae: Brachyura) were recorded during this study. A total of 691 individuals (400 males and 291 females) of this crab were collected from the intertidal coasts of the Egyptian Red Sea, Gulf of Suez and Gulf of Aqaba, of them 38 (23 males and 15 females) were infected with this parasite. The overall infection rate recorded 5.50% for all populations and was slightly higher in males (5.75%) than in females (5.15%). It showed seasonal, spatial and sex variations, recorded the highest rate (7.91%) at Gulf of Suez, declined sharply to 3.85 % at Gulf of Aqaba, and 2.43 % at all populations of the Red Sea, recorded the minimum rate of 0.57% at the southern populations. Autumn has the highest rate (10.34 %) at Hurghada (northern Red Sea), followed by summer with 9.34 % and 7.69 % at the Gulf of Suez and Ras Mohammed (Northern Red Sea), respectively, declined to 2.94 % in summer at Gulf of Aqaba, but increased again to 5.56% during spring. A total of 41 externae were recorded on the infected individuals, comprised 35 individuals with single externa (92.11 %), and only three with double externe (7.89 %). The highest number of externae was 13 (31.70 %) occurred on the 6th abdominal segment, followed by 11 (26.82 %) on the 5th segments, declined to 1-5 on the other segments except the first. The size of externae varied from 1.0 to 10.4 mm in breadth, averaged 5.19± 2.76 mm in males and 5.63± 3.17 mm in females. The rootlets of internae of the parasite invaded ovaries, testes, hepatopancreas, and all spaces within the crab body cavities. The disappearance or destruction of testes in infected males accompanied by remarkable broadness and segmentation of abdomens fringed with dense and length setae lead to “Parasitic castration”, compared with a hyperfiminzation in infected females due to the destruction of undeveloped ovaries and increasing abdominal setae dense and length.

Molecular and morphological methods are used to describe Lernaeodiscus rybakovi, a new rhizocephalan species parasitizing the porcellanid crab Pachycheles stevensii Stimpson, 1858, collected in Russian waters of the Sea of Japan. Molecular analysis of three species, including the new one, confirms the monophyly of the genus Lernaeodicus Muller, 1862 and its recent transfer to the family Peltogastridae. The main morphological features of the new species are also common with characters of the other species of the genus Lernaeodiscus Muller, 1862. Externa of L. rybakovi differs from the well studied Lernaeodiscus porcellanae from the E. Pacific by molecular markers, color and the absence of pronounced marginal lobes. Retinacula on the internal cuticle of L. rybakovi are found in the genus Lernaeodiscus for the first time. Some of the female hosts with adult externae were unusual in also carrying their own eggs on the pleopods, a rare situation among rhizocephalans. The complete larval development in the genus Lernaeodiscus is described here for the first time and includes five naupliar and one cypris instar. The main morphological features of Lernaeodiscus nauplii (the presence of flotation collar, morphology of the frontolateral horns and furcal spines, and the arrangement of dorsal setae on the shield head) are common with those of other known peltogastrid larvae, but also resemble nauplii of Peltogasterella in the presence of naupliar eyes and thin structure of the flotation collar. The sizes of male and female cyprids of L. rybakovi overlap slightly. In summer months, larval sex ratio is male-biased. We briefly review important larval characters in the Rhizocephala.

The male larvae of many parasitic barnacles are planktonic and are seasonally released. To achieve reproductive success, a male must be accepted by a receptive female that has successfully infected a host. To understand the seasonality of the breeding biology of parasitic barnacles, we developed an evolutionary game theoretical model for the seasonal pattern in the production of male larvae. Assumptions are that female parasitic barnacles become receptive following a given seasonal pattern. The parental females (mothers) choose the timing of producing their own male larvae to achieve maximum reproductive success. In the evolutionarily stable seasonal pattern, the production of male larvae often shows a sharp peak on a single day, indicating strongly synchronized production of male larvae, even when the supply of receptive females is distributed over the breeding season. When the total number of male larvae is large, the evolutionarily stable male production pattern may include multiple peaks, but it never shows a continuous distribution. This is very different from the game model previously developed for the emergence pattern of butterflies, where evolutionarily stable male emergence is always continuously distributed over a fraction of the mating season. As planktonic larvae, male parasitic barnacles have a naturally limited ability to find receptive females, and females may stay receptive for many days, whereas in butterflies, newly emerged females are mated within a day of their emergence.

Molecular and morphological methods are used to describe the rhizocephalan Lernaeodiscus kasyanovi sp. nov. from Russian waters of the Sea of Japan, the second of two Lernaeodiscus species parasitizing the porcellanid crab Pachycheles stevensii Stimpson, 1858. Lernaeodiscus kasyanovi sp. nov. differs from the other species found on this host, Lernaeodiscus rybakovi Korn et al., 2020a, by molecular markers, by a smaller size, a lighter color, by the shape of receptacles, by the presence of marginal lobes in mature specimens, and by the retinacula type on the internal cuticle. Lernaeodiscus kasyanovi sp. nov. is the sister taxon to Lernaeodiscus porcellanae in the monophyletic genus Lernaeodiscus. Lernaeodiscus kasyanovi is rather rare compared to L. rybakovi, their prevalence on the porcellanid crab P. stevensii does not exceed 2%. It is the second example of two rhizocephalan species infesting the same brachyuran species in Peter the Great Bay.

The present investigation is the first in Egypt to give histomorphological data about Heterosaccus dollfusi, a rhizocephalan parasitic barnacle of the brachyuran crab Charybdis longicollis, collected from the eastern Mediterranean coast of Egypt at shallow depths (10–20 m). The structure of the mature externa and its male organs are essential for perception of both the taxonomy and reproductive biology of these specialized parasites. We use scanning electron microscopy and histological methods coupled with morphological description to investigate the anatomy of mature externa. The mature externa is distinguished by a visceral mass which comprises the ovary, paired colleteric glands, paired male receptacles and paired receptacle ducts. Attention is focused on the developmental stages of male organs and/or tracing of spermatozoa after settlement of male cypris implantation of the trichogon during fertilization process. For the first time, the lateral tube that connects the left male receptacle with the right, smaller receptacle was observed in sagittal section and illustrated in the diagrammatic figure of male organs. This tube is probably the generator of sperm ribbons. As well, the existence of the spermatozoa inside the lumen of this connecting tube may indicate their free movement between the two receptacles. Moreover, strips of sperm that drag from the sperm ribbon ball, and surround the developing ovules can also be seen in the mantle cavity.

FAMILY POLYASCIDAE H��EG & GLENNER, 2019 Diagnosis: As provided by H��eg et al. (2020). Comment: This family was recently separated from the Sacculinidae by molecular evidence. Future analyses might well see the transfer of additional species from the Sacculinidae to the Polyascidae. Host: Brachyura and Gebiidea. Parasacculina H��eg & Glenner, 2019 (five species) Polyascus Glenner, L��tzen & Takahashi, 2003 (three species), Published as part of Chan, Benny K. K., Dreyer, Niklas, Gale, Andy S., Glenner, Henrik, Ewers-Saucedo, Christine, P��rez-Losada, Marcos, Kolbasov, Gregory A., Crandall, Keith A. & H��eg, Jens T., 2021, The evolutionary diversity of barnacles, with an updated classification of fossil and living forms, pp. 789-846 in Zoological Journal of the Linnean Society 193 on page 826, DOI: 10.1093/zoolinnean/zlaa160, http://zenodo.org/record/5637275, {"references":["Hoeg JT, Rees DJ, Jensen PC, Glenner H. 2019. Chapter 9. Unravelling the evolutions of the Rhizocephala - a case study for molecular based phylogeny in the parasitic Crustacea. In: Smit NJ, Bruce NKL, Malherbe K, eds. Parasitic crustacea: state of knowledge and future trends. Berlin: Springer, 387 - 419.","Hoeg JT, Noever C, Rees DA, Crandall KA, Glenner H. 2020. A new molecular-based taxonomy of the parasitic barnacles (Crustacea: Cirripedia: Rhizocephala). Zoological Journal of the Linnan Society 190: 632 - 653."]}

We present a comprehensive revision and synthesis of the higher-level classification of the barnacles (Crustacea: Thecostraca) to the genus level and including both extant and fossils forms. We provide estimates of the number of species in each group. Our classification scheme has been updated based on insights from recent phylogenetic studies and attempts to adjust the higher-level classifications to represent evolutionary lineages better, while documenting the evolutionary diversity of the barnacles. Except where specifically noted, recognized taxa down to family are argued to be monophyletic from molecular analysis and/or morphological data. Our resulting classification divides the Thecostraca into the subclasses Facetotecta, Ascothoracida and Cirripedia. The whole class now contains 14 orders, 65 families and 367 genera. We estimate that barnacles consist of 2116 species. The taxonomy is accompanied by a discussion of major morphological events in barnacle evolution and justifications for the various rearrangements we propose.

Bopyrid isopods and rhizocephalan barnacles are obligate parasite crustaceans which harm their decapod hosts. However, to the best of our knowledge, studies have not compared which of these parasites has a greater parasitic effect on its hosts. Here, the parasitic effect of the bopyrid isopod, Allokepon hendersoni, and an unidentified sacculinid rhizocephalan species, infesting the same population of portunid crabs, Charybdis bimaculata, was investigated and compared for the first time. Samples were collected from the bycatch of a trawl fishery in Tosa Bay, Japan. A total of 2601 crabs were collected, of which 14 (0.55%) were parasitized by the bopyrid and 21 (0.82%) by the rhizocephalan. One of the two female crabs parasitized by the bopyrid was ovigerous (with much fewer eggs than unparasitized females). No ovigerous crab was found from the eight females parasitized by the rhizocephalan. Because only two female crabs were parasitized by the bopyrid, the following analyses were made using the male crabs. Both parasites reduced the wet weight (crab condition) and the cheliped size (secondary growth) of C. bimaculata, but the impact of the parasitism did not differ between the parasite species. The size of the abdominal flap of male hosts was reduced by the bopyrid infestation; however, rhizocephalan infestation caused enlargement of the abdominal flap, which is an indication of feminization. The present study provides information on how the effect of these two parasitic castrators on the same host crab varies. A moderate decrease in crab condition and cheliped development was common among the parasites, suggesting that the degree of impact might be favorable for the survival of the two parasites species.

The reproduction and parasite associates of the squat lobster Munidopsis platirostris (A. Milne Edwards and Bouvier, 1894) were investigated based on collections made in the Bahamas and Curacao wit...

A rhizocephalan parasite of the hermit crab Pagurus minutus Hess, 1865 (Decapoda, Paguridae) is described from Russian waters using morphological and molecular methods. Partial sequences of the mitochondrial cytochrome c oxidase subunit I gene (COI) were used as a genetic marker. We refer this species to Peltogaster reticulata Shiino, 1943 (Rhizocephala, Peltogastridae) based on its morphological description, since genetic data for the species from its type locality are lacking. The species can be distinguished from its known congeners by the reticular pattern of the external cuticle. Peltogaster reticulata is the sister taxon to P. postica Yoshida & Osawa, 2011 (in Yoshida et al. 2011), inhabiting the same host in southwestern Japan. The complete larval development of Peltogaster reticulata, including five naupliar and one cypris stage, is described and illustrated using SEM.

A sacculinid rhizocephalan parasite,Sacculina lataBoschma, 1933, was discovered in the Beibu Gulf (= Gulf of Tonkin) west of Hainan Island, China, occurring in 7.4% of 363Charybdis miles(De Haan, 1835) collected by 30 trawls in July 2017. This is the first time thatS. latahas been recorded in Beibu Gulf and in the Chinese Exclusive Economic Zones (EEZs). Of the sampled crabs from 9 stations, 8 males and 19 females were found bearing externae ofS. lataon the abdomen, a prevalence of 4.1 and 11.2% in male and female crabs, respectively. The dominant size group of infected males was larger than that of females. There was no significant relationship between water depth and infection rate of the parasite. A positive correlation was found both between volume and weight of externae, and between surface area of externae and abdomen width of the host crabs. Carapace width and wet weight of infected crabs were significantly less than those of uninfected crabs. In male crabs, the first pleopods of infected individuals were shorter than normal, but their pleons were wider. The propodus of the chela of a parasitized crab was significantly larger than that of normal crabs, both in males and females. No rhizocephalan-infected female crab was found bearing eggs.

Population and developmental characteristics of the rhizocephalan parasite Sacculina carcini were studied in the Limfjord, Denmark and at Roscoff, France. The frequency of juvenile externae...

In dynamic systems, organisms are faced with variable selective forces that may impose trade-offs. In estuaries, salinity is a strong driver of organismal diversity, while parasites shape species distributions and demography. We tested for trade-offs between low-salinity stress and parasitism in an invasive castrating parasite and its mud crab host along salinity gradients of two North Carolina rivers. We performed field surveys every six to eight weeks over 3 years to determine factors influencing parasite prevalence, host abundance, and associated taxa diversity. We also looked for signatures of low-salinity stress in the host by examining its response (time-to-right and gene expression) to salinity. We found salinity and temperature significantly affected parasite prevalence, with low-salinity sites (less than 10 practical salinity units (PSU)) lacking infection, and populations in moderate salinities at warmer temperatures reaching prevalence as high as 60%. Host abundance was negatively associated with parasite prevalence. Host gene expression was plastic to acclimation salinity, but several osmoregulatory and immune-related genes demonstrated source-dependent salinity response. We identified a genetic marker that was strongly associated with salinity against a backdrop of no neutral genetic structure, suggesting possible selection on standing variation. Our study illuminates how selective trade-offs in naturally dynamic systems may shape host evolutionary ecology.

Pathophysiological studies of rhizocephalan infections are rare. We describe differences in the levels of tissue and hemolymph metabolites between Polyascus plana-parasitized and unparasitized individuals of Metopograpsus thukuhar. Crabs were assigned to either a parasitized (carrying at least 1 externa, i.e. a protruding reproductive body) or an unparasitized (not carrying externae and determined to be rootlet-free by a barnacle 18S rRNA-based polymerase chain reaction) group. Quantification of metabolites showed that muscle glycogen levels were significantly lower and hepatopancreas levels were significantly higher in parasitized crabs compared to unparasitized crabs; hepatopancreas triacylglycerol levels were significantly higher and hemolymph levels significantly lower in parasitized hosts, and there was no significant difference in muscle triacylglycerol levels between unparasitized and parasitized animals. Glucose levels in the hepatopancreas, muscle, and hemolymph were all significantly higher in parasitized hosts. Significant levels of glucose, triacylglycerol, and glycogen were present in the barnacle externae. In addition, levels of crustacean hyperglycemic hormone in the sinus glands were not significantly different between unparasitized and parasitized animals. Glucose mobilized from the muscle is likely converted to glycogen and triacylglycerol in the rootlet-infiltrated hepatopancreas of parasitized hosts, and the eyestalk neuroendocrine system appears not to be significantly impaired, in terms of hormone production and storage, by parasitization.

Parasitic rhizocephalan barnacles induce morphological, physiological, and behavioural changes in their hosts. The mechanisms of these intimate host-parasite interactions remain unknown. We have shown previously that rootlets of the internae of Peltogasterella gracilis and Peltogaster paguri penetrate the ganglion's envelope of their hermit crab hosts and form specialised structures in the ganglion periphery, the so-called goblet-shaped organs. Here, we examine the gross morphology and ultrastructure of these goblet-shaped organs in the interna of Sacculina pilosella. They consist of three layers of cells; in the intermediate layer of the organs, unusual lamellar bodies and muscle cells were found. Extensive degeneration of the host nervous tissue was observed in the funnel of the goblet-shaped organs. We conclude that the ability to penetrate into the host's nervous tissue could be a common trait in rhizocephalans. The goblet-shaped organs may play a key role in the host-parasite relationships by enabling the parasite to influence the host via hormones and neurotransmitters.

Crustaceans play essential roles as food sources for global coastal communities, and their fisheries and aquaculture support the economic growth of many coastal countries. Thus, the understanding of diseases in crustaceans, especially those with high economic values, are important for their cure and prevention. Among other parasites that parasitise crustaceans, parasitic barnacles (Rhizocephala) are considered the most intriguing, both in terms of their unique life cycle and the detrimental changes they induced upon their hosts such as host sterilisation, moult inhibition (anecdysis), growth reduction, feminisation of male individuals, and behaviour alteration. In addition to discussing in detail the morphological, physiological and behavioural changes caused by rhizocephalan infestations, this review also shows that rhizocephalans are found in most economically important crustacean species around the world. Based on their reported adverse effects on their hosts, their potential implications on various aspects of crustacean aquaculture including broodstock selection, culture and fattening, soft-shell crab production and breeding programs are discussed. Although treatment or cure for rhizocephalan infestation has yet to be found, several recommendations are provided to enable accurate identification and handling of infected hosts and exposed aquaculture facilities in order to minimize losses and prevent parasite outbreaks.

Microsporidians, peritrich ciliates, udonellids, tantulocarids and parasitic isopods have been found associated with parasitic crustaceans, i.e. Branchiura, parasitic Copepoda and Isopoda, as well as Rhizocephala (parasitic Cirripedia). Information on the Microsporida found in parasitic Copepoda are scarce, whilst infestation in gregarines, myxosporidia, cestodes and trematodes has been reported. Information is provided from known records of hypersymbiont infestations, as well as some unpublished data collected of peritrichs (Epistylis and Vorticella) found on the fish parasites Dolops and Argulus (Branchiura), members of Lernaea and Opistolernaea (Copepoda: Lernaeidae) and Ergasilus (Copepoda: Ergasilidae). The genus Doropygus (Copepoda), found in the branchial chamber of redbait (Ascidiacea), also harbours peritrichs. These hypersymbionts take the association to the next level, as the ciliates themselves harbour zoochlorellae. Helminths of the family Udonellidae are found associated with caligids and sometimes branchiurans, which occur on the skin of marine fishes. Tantulocaridans (Hypertantulus siphonicola) have been reported from siphonostomatoid copepods. Cabiropidae (Cabirops) are hyperparasites found in the brood pouch of bopyrid hosts, and C. orbionei might be considered as biological control for bopyrids, which are found on penaeid shrimps. Four genera of the family Cryptoniscidae (Liriopsis, Cryptoniscus, Hemioniscus and Danalia) and one genus of the family Cabiropidae (Perezina) are parasites of rhizocephalans that are parasitic on decapod hosts.

A series of specimens of Malacoplax californiensis (Lockington, 1877) is reported from the Gulf of California, significantly increasing the known localities for the otherwise rarely reported species from 3 to 12. Depth range is increased to 110‒114 m on the continental shelf. Environmental conditions associated with the crabs were: 12.4‒27.0°C; 1.9‒5.0 ml/l O2; 58‒98% sandy sediments. Males and females were sexually distinct even at small sizes (4.2 mm and 3.5 mm CW, respectively). Two small specimens were infested with a rhizocephalan, a condition not reported previously for this species of panopeid.

We use Rhizocephala to illustrate the problems inherent in estimating the phylogeny of parasitic crustaceans. The adult rhizocephalan parasite has such a reduced morphology that little else than the presence of a moulted cuticle relegates them to Arthropoda. Therefore, until the advent of molecular phylogenetic analysis relegation of the Rhizocephala to taxon relied exclusively on larval characters. The reduced adult morphology also entailed that very few characters were available for intrinsic rhizocephalan systematics and virtually none that could be compared with any outgroup. Thus, rhizocephalan taxonomy relied only on analysis within the group and, with few exceptions, was not based on any phylogenetic principles. The advent of DNA methods in phylogeny confirmed, with high confidence, rhizocephalans as cirripedes and nested them within the taxon as the sister group to Thoracica. This result was supported by SEM studies of the cypris larvae. Additional molecular phylogenetic studies yielded detailed insight into rhizocephalan relationships, culminating in the phylogeny presented here, which includes species from all but one family. Contrary to traditional hypotheses, the new phylogeny shows that infection of the host using a kentrogon stage represents the plesiomorphic condition, while rhizocephalans (Akentrogonida) without this stage are advanced. In this analysis, both Kentrogonida and the species-rich Sacculinidae are paraphyletic. In addition, recent hypotheses on family-level relationships in Akentrogonida, based on larval structure and the sexual system, were largely confirmed. This shows that when accurately analysed in a strict homology regime, morphological characters can be powerful partners to molecular data in elucidating rhizocephalan phylogeny.

Crustaceans have independently adopted a parasitic mode of life on numerous occasions, and this chapter reviews the classification and species richness of each of the parasitic clades, which lie within two major pancrustacean lineages, the Multicrustacea and the Oligostraca. Tabulated data are presented on the genera, species richness and host usage of the whale lice (Cyamidae); the generic and species richness of the families of hyperiidean amphipods; the generic and species richness and the host usage of the families and subfamilies of epicaridean isopods; the species richness, salinity regime and host taxon of cymothoid isopods; the generic and species richness and the host usage of the families of Ascothoracida and Rhizocephala; the species of Tantulocarida and their hosts; the generic and species richness and the host usage of the families and family-level groupings of cyclopoid and siphonostomatoid copepods; the species richness and salinity regimes of the genera of Branchiura; and the species richness and host usage of the genera of tongue worms (Pentastomida). Parasitic crustaceans use a total of 15 different phyla as hosts.

Rhizocephalan barnacles have been reported to parasitize a wide range of king crab species (Lithodidae). So far all these parasites have been assigned to a single species, Briarosaccus callosus Boschma, 1930, which is assumed to have a global distribution. Here we investigate Briarosaccus specimens from three different king crab hosts from the fjord systems of Southeastern Alaska: Lithodes aequispinus Benedict, 1895, Paralithodes camtschaticus (Tilesius, 1815), and Paralithodes platypus (Brandt, 1850). Using molecular markers and by morphological comparison we show that Briarosaccus specimens from these three commercial exploited king crabs are in fact morphologically distinct from B. callosus, and further represent two separate species which we describe. The two new species, Briarosaccus auratum n. sp. and B. regalis n. sp., are cryptic by morphological means and were identified as distinct species by the use of genetic markers (COI and 16S). They occur sympatrically, yet no overlap in king crab hosts occurs, with B. auratum n. sp. only found on L. aequispinus, and B. regalis n. sp. as parasite of the two Paralithodes hosts.

Specimens of the commercially important crab Portunus sanguinolentus (Herbst, 1783) were collected monthly in Honghai Bay, P. R. China and examined for the presence of two barnacles: the Lepadomorpha Octolasmis bullata (Aurivillius, 1894) and the Rhizocephala Diplothylacus sinensis (Keppen, 1877). O. bullata was observed in the branchial chambers and prevalence of their infestation varied in the monthly collections. High prevalence occurred in the time from June to August and from October to February. The mean intensity of O. bullata was 23.33 ± 18.91 barnacles per infested host, showing higher intensity in larger crabs. The parasite D. sinensis was observed on host surfaces with externae or scars, and their prevalence was high in winter and low in summer. Crabs parasitized by D. sinensis showed a heavier fouling by O. bullata than the non-parasitized crabs. Larger crabs were more vulnerable to the infestation of O. bullata and the parasitism of D. sinensis. Thus, interaction of O. bullata and D. sinensis may cause synergistic effects on the host’s health. Due to the great economic importance of the crab, attention should be paid to this relationship.

Peltogasterella sensuru n. sp. infests Pagurixus hermit crabs inhabiting rocky shores off Okinawa Island (Ryukyu Archipelago, Japan). This species is clearly distinguishable from Peltogasterella gracilis (Boschma, 1927): the stalk emerges from the middle part of the externa in the present species and P. sulcata (Lilljeborg, 1859), while the stalk in P. gracilis emerges from the posterior end of the externae. The new species differs from P. sulcata based on the morphology of the mantle aperture. Peltogasterella sensuru n. sp. repeatedly produces single brood externae that have not been previously observed in species belonging to the suborder Kentrogonida Delage, 1884. We also determined partial sequences of the COI gene and 16S rRNA gene of the new species for use as molecular markers for species identification.

Parasitism by rhizocephalans can cause morphological, physiological, and behavioral changes in their host crabs, thus potentially also affecting the structure and function of host populations in the benthic ecosystem. The dynamic variation in crab populations caused by rhizocephalans may affect the colonization of epibionts on the crabs, but there is little literature on host/parasite/epibiont relationships. This study investigated the patterns of colonization by the thecostracan barnacle Octolasmis on crabs parasitized by the rhizocephalan externae of Diplothylacus sinensis, in order to quantify the interaction of the epibionts and rhizocephalan. Two species of epibiont barnacles, Octolasmis bullata and O. wanwickii, occurred on the host crab species Portunus sanguinolentus: O. bullata was found in the branchial chambers of all rhizocephalan-parasitized crabs, with an intensity of 74.6 +/- 39.1 (mean +/- SD) barnacles per host; O. warwickii was found attached to the outer surfaces of 34.6% of parasitized crabs, with an intensity of 49.3 +/- 32.3 barnacles per host. Their patterns of colonization, including Octolasmis prevalence, intensity, and distribution, were not similar to those crabs unparasitized with the rhizocephalan externae in previous reports. Differences between the two groups of crabs were closely related to the parasitism by rhizocephalan externae of D. sinensis. Because the evidence in this study suggests that the parasite had arrested or reduced a series of host behaviors, such as molting, grooming, and burying, to protect the parasitization of rhizocephalan externae, although these behavioral patterns are natural antifouling mechanisms for the epibionts. As a result, Octolasmis could benefit from the behavioral changes of hosts in the rhizocephalan-crab association. Whereas, massive colonization of Octolasmis on rhizocephalan-parasitized crabs, especially long-term parasitized hosts, could pose a severe burden and lead to an increase in host mortality.

Rhizocephalans of the order Akentrogonida are parasitic barnacles that infest decapods, stomatopods, peracarids, and other cirripedes. Within this order, Thompsoniidae is found in decapods and stomatopods, and is a family comprised of species characterized by having multiple globular reproductive bodies (externae) erupting from the host’s extremities and abdominal surface, and connected internally by a root-like system. Two of the four genera currently accepted within Thompsoniidae, Thompsonia and Thylacoplethus, contain perhaps the most morphologically derived species within Akentrogonida. Herein we describe two new species of Thylacoplethus, T. porcellanus and T. umanguvatus, from the porcellanid and hermit crabs Petrolisthes scabriculus (Dana, 1852) and Calcinus morgani Rahayu and Forest, 1999, respectively. We also redescribe Thompsonia japonica Hafele, 1911 from a xanthid brachyuran crab based on morphological characters and DNA sequences of the nuclear 18S rDNA gene. The new species from the porcellanid crab comprises the first record of an akentrogonid rhizocephalan parasitizing a species of Porcellanidae. We reconstruct a molecular phylogeny of Akentrogonida using new and published DNA sequences, and we discuss our findings within an evolutionary context of host speciation. All species in the four thompsoniid genera are briefly reviewed, and a replacement name is proposed for the preoccupied Pottsia Hoeg and Lutzen, 1993.

All available biological, ultrastructural, and molecular data support the conclusion that the cirripedian suborders, Rhizocephala and Thoracica, are closely related. Williamson’s speculations (Williamson, D. I., 2014. The origin of barnacles (Thecostraca, Cirripedia). Crustaceana,87: 755-765) that the so-called hybridogenesis can explain how Rhizocephala evolved, rest on a selective choice of other authors’ data and an ignorance of solid facts derived from decades of comparative anatomical and molecular studies.

A new species of rhizocephalan barnacle is described parasitizing the Florida lobsterette Nephropsis aculeata Smith, 1881. This is the first record of a nephropoid as a host for any rhizocephalan and represents the first record of a rhizocephalan parasitizing any marine lobster (Achelata, Astacidea, Glypheidea, Polychelida). The new species belongs to the genus ParthenopeaKossmann, 1874, which now contains three species: Parthenopea australisLutzen, Glenner & Lorz, 2009 (on a callianassid in New Zealand), P. subterraneaKossmann, 1874 (on a callianassid and an axiid in the Skagerrak strait, Black Sea, and Mediterranean), and P. reinhardi n. sp. (on N. aculeata from Dry Tortugas, Florida, USA). The new species can be distinguished from the other two in the genus based on the shape of the seminal receptacle ducts (proximally straight in P. subterranea versus coiled in P. reinhardi n. sp.) and annuli on the stalk (present in in P. australis but absent in P. reinhardi n. sp.) and the position of the mantle opening (on the surface and close to the stalk in P. australis versus in a groove and more distant from the stalk in P. reinhardi n. sp.). We urge researchers and fisheries scientists working on marine lobsters to examine specimens for rhizocephalans, potentially leading to additional new records among lobster hosts.

A thompsoniid rhizocephalan parasite, Diplothylacus sinensis (Keppen, 1877), was discovered in the semi-open Honghai Bay in the South China Sea, occurring in 26.4% of 1852 (934 males and 918 females) Portunus sanguinolentus (Herbst, 1783) (Decapoda: Portunidae) collected monthly between November 2012, and October 2013. Of the sampled crabs, 243 males and 246 females were infected with externae of D. sinensis or bore attachment scars on the body surface, giving an overall prevalence of infection of 26.0% and 26.8% in male and female crabs, respectively. The infection rate changed monthly with the highest prevalence in December (50.6%) and the lowest in June (1.3%). New externae filled with eggs or embryos were observed throughout the year, which indicates a year-round reproductive cycle, but virgin externae were more commonly present in late spring or early summer. Carapace width and wet weight of infected crabs of both sexes were significantly smaller than those of uninfected crabs. Compared to normal crabs, the pleon of infected females was significantly wider and the first pleopod of parasitized males was shorter. On addition, crabs infected by the rhizocephalan parasite were more susceptible to other epibionts, which included Octolasmis warwickii Gray, 1825 (10.1%), Chelonibia patula (Ranzani, 1818) (6.8%), Obelia spp. (7.2%) and Membranipora membranacea (6.5%) on the carapace surface, and Octolasmis bullata (Aurivillius, 1894) (64.7%) on the gills.

Of 2282 individuals in 23 hermit crab species collected from four coasts of the southern part of mainland Japan along the Pacific coast, 87 individuals in seven species were infested by six rhizocephalan species. Among the ten combinations between hosts and rhizocephalans, seven combinations were new records. Morphological identification of the rhizocephalan species from different host species and/or sites was consistent with molecular identification using DNA sequences (COI and 16S rRNA genes). Pagurus filholi and P. nigrivittatus were infested by both Peltogaster postica and P. lineata. The host preference of the two rhizocephalan species reciprocally shifted from one species to the other among the four collection sites. Rhizocephalans may choose the host inhabiting the preferred environment for the parasite within the range of their host species.

I claim that hybridogenesis has played a vital part in the evolution of the Thoracica, Acrothoracica, Rhizocephala and Facetotecta. Hybridogenesis is the generation of new life forms and new life histories by sexual hybridization. It includes larval transfer, which explains the origins of all larvae, and component transfer, which explains the origins of cnidarians, lophophorates and thoracicans. I propose that adult thoracicans are descended from a hybrid between a crustacean and a non-arthropod, the genomes of which are expressed together. They are concurrent chimeras. Adult acrothoracicans are equivalent to the crustacean part of thoracicans. Nauplius larvae were added to the life histories of thoracicans and some acrothoracicans by later hybridizations. Adult rhizocephalans and juvenile facetotectans have no arthropod characteristics, and I claim that both these taxa acquired nauplius and cypris larvae by hybrid transfer. Extreme differences between the mitochondrial genomes of a sacculinid rhizocephalan and thoracicans are consistent with this view of rhizocephalan evolution. The Rhizocephala and Facetotecta are not members of the Cirripedia or the Arthropoda.

Complete 18S rDNA sequences of two species of the Tantulocarida Arcticotantulus pertzovi (Basipodellidae) and Microdajus tchesunovi (Microdajidae) were obtained and used for estimating the relationship of the class with other Crustacea. This constitutes the first use of tantulocaridan gene sequences, and we conclude that the Tantulocarida are very close relatives of the class Thecostraca, which comprise cirripedes, ascothoracidans and the enigmatic facetotectans. With much lower confidence, the Tantulocarida are also indicated as nested within the Thecostraca, being sister group to the Cirripedia. We therefore discuss morphological similarities and differences between tantulocaridans and the thecostracans in search of potential synapomorphies, including a possible relation to the parasitic barnacles (Rhizocephala). We conclude that the cement gland of the tantulus larva and the cirripede cyprid might be homologous structures, but that similarities in host infection and root systems between the Tantulocarida and the Rhizocephala are, on present evidence, likely to be homoplasies evolved by convergent evolution into advanced parasitism. The precise position of the Tantulocarida in relation to or within the Thecostraca must be pursued by a more extensive database of genetic markers.

Data on the external morphology are reviewed and new information is provided about the structure of the first naupliar instar of Parthenopea subterranea Kossmann, a rare and poorly known species of rhizocephalan barnacles. The taxonomic position of P. subterranea, which has so far been uncertain, is discussed. A new family, Parthenopeidae fam. nov., is erected to include a single genus, Parthenopea, with a single species, P. subterranea. The distinctive characters of the new family are described.

Parasites generally influence the feeding behavior of their host and may therefore indirectly impact ecosystem structure and functioning if the host plays an ecological key role. The ecologically important shore crab ( Carcinus maenas ) is commonly infected by the rhizocephalan parasite Sacculina carcini that aside from inflicting behavioral change, castration and ceased molting, also feminizes its male host morphologically. The latter results in reduced cheliped size, and, together with the other parasite-induced effects, this may potentially impact host feeding behavior. In two separate laboratory experiments, we offered infected and uninfected adult male crabs respectively ad libitum small, easy-to-handle blue mussels ( Mytilus edulis ) (10–15 mm in shell-length), and a limited, size-structured prey population (15–45 mm in shell-length; seven size-classes, ten mussels per class) during 10–15 days. Corrected for carapace width, the per capita consumption rate of the infected and uninfected crabs was similar in either experiment, both regarding number of mussels and amount of tissue dry-weight consumed. Also, the median mussel size preyed upon when exposed to the size-structure prey population was unaffected by infection. However, infected crabs preyed less frequently (26%) on intermediate mussel sizes (25–30 mm) than uninfected crabs. For both infected and uninfected crabs the median prey size increased linearly with maximum claw height. Host dry weight was significantly reduced by infection, assumed to be the result of the morphological feminization (reduced appendage size) rather than reflecting poorer general condition of infected individuals. Infected crabs were nonetheless subjected to a higher mortality rate than uninfected crabs during the experimental period. We conclude that Sacculina infection has a very limited effect on its host crabs' feeding biology and that any measurable ecosystem ramifications of the host–parasite association must originate from other processes; for instance reduced mean size (infection inhibits molting) and density (infection increases mortality) of the crab population where parasitism is high.

The species of Cabiropidae having a Cabirops-type morphology of the mature female (lateral body compression) are reviewed, including members of the genera Cabirops Kossmann, Arcturocheres Hansen, and Bourdonia Rybakov, all of which parasitize isopods. The genus Astacilloechus Hansen is shown to be a synonym of Arcturocheres, and Paradajus Nierstrasz & Brender a Brandis is a synonym of Cabirops. Danalia fraissei Nierstrasz & Brender a Brandis is transferred to Cabirops. A new genus, Rolandoniscus, is erected for Cabirops serratus Bourdon. The rhizocephalan parasite Perezina Nierstrasz & Brender a Brandis is confirmed to belong to Cabiropidae, not Cryptoniscidae. A key to genus-groups, based on female morphology, of the 12 genera within Cabiropidae is provided, as is a key, based on the cryptoniscus stage, to the five genera with Cabirops-type female morphology.

Parasitism is increasingly recognized as an important factor that can influence the structure and function of natural communities. The presence of externae of the parasite Sacculina carcini Thompson, 1836 was investigated in a population of the crab Carcinus maenas (Linnaeus, 1758) from the Mondego estuary, Portugal. A seasonal pattern of the S. carcini externae presence was observed with the highest values recorded in winter and the lowest in summer, ranging from 0.6 to 10%. The parasite seems to affect more females than males and the red morphotype presented higher prevalence than the green one. The size class [25, 35 mm[ was the one with the highest prevalence. The mouth of the estuary was the area where the most externally sacculinized crabs were caught. Apparently, the presence of the parasite tends to promote crabs migration to the mouth. This area coincides with the preferential spawning local of non-parasitized females. The infection by S. carcini influences the crab’s demography and its life history.

A new rhizocephalan species, Peltogaster unigibba n. sp., is described from the host hermit crab, Pagurixus boninensis (Melin, 1939), from the Bonin Islands, Japan. Of the16 known species of Peltogaster now currently recognised, P. unigibba n. sp., and P. contorta Boschma, 1958 share a left lobe that projects beyond the mantle aperture. The two species can be distinguished from one another by the position of the opening of the mantle aperture. The new species most closely resembles P. naushonensis Reinhard, 1946 in its internal structure, but clearly differs in the relative length of the colleteric glands. Peltogaster unigibba n. sp. represents the first record of a rhizocephalan from the oceanic Bonin Islands, and the second record of a rhizocephalan from an oceanic island in the northern hemisphere.

Upon infesting the hermit crab Pagurus proximus the parasitic barnacle Peltogaster reticulatus typically forms a single externa on each host. In vivo, histological and histochemical study revealed that most P. reticulatus with a single mature externa also exhibited three or four primordial externae and several buds of earlier developmental stages. It is shown that stolon buds and primordial externae in P. reticulatus internae intensively express alkaline phosphatase activity, the classical histochemical marker for mammalian embryonic stem and primary germ cells. Ovaries of the most developed primordial externae were filled with growing oocytes, so the process of oogenesis begins in the externa rudiments of P. reticulatus before the externae appear on the surface of the host body. Since the interna, which is connected to the mature externa, also integrated three or four additional primordial externae and several earlier buds, P. reticulatus, a non-colonial species based on the common presence of a single externa in each host, appears to have a hidden colonial organization at the parasitic stage of its life cycle.

A new genus and two new species of Peltogastridae, Peltogaster postica sp. nov. and Ommatogaster nana gen. et sp. nov., are described from Okinawa Island, Ryukyu Islands, southwestern Japan. The two new rhizocephalans were found to be parasitic on the estuarine hermit crabs, Pagurus minutus Hess, 1865 and Diogenes leptocerus Forest, 1956, respectively. Peltogaster postica sp. nov. is allied to P. curvata Kossmann, 1874, P. paguri Rathke, 1842 , and P. reticulata Shiino, 1943 , but is distinguished by its relative length and internal and external structures of the mature externa. Ommatogaster gen. nov. is established for the present new species O. nana based on the morphologies of the visceral mass of the externa and the presence of a nauplius eye in the larvae. Partial COI sequences were obtained from the two new species and one known species, Dipterosaccus indicus Van Kampen and Boschma, 1925, to test the possible usefulness of the sequences as tags for species identification.

Host specificity is a key variable of the niche breath of parasites that can be an important determinant of a parasite’s ability to invade new areas. There is increasing evidence that many parasite species may comprise a variety of genetically variable lineages, which differ in host specificity and geographic range. In this study, we (1) explored the extent of diversity in the invasive parasitic barnacle Loxothylacus panopaei (Rhizocephala) infecting mud crabs (2) examined the geographic origin for the invasive lineage and (3) assessed if further southward spread of the parasite may be impeded. Along the US Atlantic coast, L. panopaei infects different hosts in its invaded range (Chesapeake Bay to north of Cape Canaveral) compared to one portion of the native range in Southeast Florida. This difference was reflected in genetic lineages on two independent loci, mitochondrial cytochrome oxidase I and nuclear cytochrome c. Both loci were concordant in that they showed one lineage infecting crabs of the genus Panopeus in the native range and one lineage infecting Eurypanopeus depressus and Rhithropanopeus harrisii hosts in the invaded range and in the Gulf of Mexico, thus indicating Gulf of Mexico populations as the most likely source of introduction into Chesapeake Bay. Interestingly, the nuclear marker resolved an additional lineage of parasites infecting panopeid hosts in the native range. All three parasite lineages were well supported, but a decision about species status must await further analyses. Since its introduction in the 1960s, the invasive L. panopaei lineage has expanded its range southward along the US Atlantic coast, now almost reaching the northern limit of native Panopeus-infecting lineages at Cape Canaveral, Florida. We hypothesized that parasite-free E. depressus in Southeast Florida, living in sympatry with infected panopeid populations, might be resistant to infection by the invasive lineage. Our infection experiments rejected this hypothesis, suggesting that any impediment to further southward range expansion might be expected from temperature regimes of the subtropical zoogeographic region south of Cape Canaveral.

Hermit crabs (superfamily Paguroidea), by their typical nature as inhabitants of mobile, protective domiciles, are a unique source for studying symbiotic relationships. A great variety of symbionts attach, bore into or live free in the lumen of shells harboring hermit crabs, and hermit crabs themselves as hosts for a variety of parasites and potential parasites are the subject of this review. Approximately 130-140 species of the > 850 known hermit crab species are parasitized by 149 species in 9 phyla, and 27 species are hosts for 17 potential parasites in 5 phyla. Among the confirmed parasites, 30 species of parasitic barnacles (Rhizocephala) and 83 species of parasitic Isopoda are known. Species in other groups of parasites are few in number: Apicomplexa 2, Dinoflagellata 1, Microsporida 2, Ascomycota 1, Platyhelminthes 8, Acanthocephala 2, Nematoda 3, and Nematomorpha 2. Fifteen species of ectosymbiotic mites are obligate associates, but little is known of their possible parasitic tendencies. In terms of trophic strategies, the rhizocephalans, nematomorphans and two flatworm species are complete parasitic castrators and the parasitic isopods are partial parasitic castrators. The rest of the species are trophically transmitted parasites (Apicomplexa, Platyhelminthes, Acanthocephala and Nematoda) or are classified as pathogens (Apicomplexa, Dinoflagellata, Microsporida, and Ascomycota). Limited information shows that hermit crabs are first or second intermediate hosts for coccidians, cestodes, trematodes, acanthocephalans and nematodes. The importance of hermit crabs as definitive and intermediate hosts is greatly underestimated due to insufficient worldwide sampling for parasites. The ten most well-studied hermit crab host species are from European waters, the western Atlantic and the eastern Pacific. In contrast, the lower parasite diversity of the Indo-West Pacific region is presumably due to limited sampling. Hermit crabs are also known to host at least 17 species of obligate, intimate associates (not presently considered parasites), but further analyses are required to more fully clarify their symbiotic relationships. Among these potential parasites are four mesomycetozoans that are found in the digestive tract of hosts and four apostome ciliates whose reproduction is synchronized with the host's molting. Symbiotic nemerteans and octolasmid barnacles that are common associates of decapods have, surprisingly, yet to be discovered associated with hermit crabs. In total, at least ten species of hyperparasites are found infesting primary associates (mostly rhizocephalans and bopyrids) of hermit crabs. Hyperparasitism involves: 1) rhizocephalans as hosts for one described species of amoeba and four species of cryptoniscid isopods, 2) bopyrid isopods as hosts for three described species of cabiropid isopods and one rhizocephalan (Akentrogonida), and 3) a burrowing barnacle (Trypetesidae) as host for one species of hemioniscid isopod. The biology and life histories of most hermit crab parasites, with the exception of rhizocephalans and some isopods, are poorly known and the role of crabs as intermediate hosts is especially in need of study. The information presented herein on geographical distribution of host species, the prevalence of their parasites, and known host-parasite relationships should be useful for developing models that will elicit a greater understanding of hermit crab parasitism. Such information, along with morphological and molecular analyses might lead to a better understanding of the evolution of anomurans and decapods as a whole.

We studied spatial variation in the prevalence of the rhizocephalan barnacle Lernaeodiscus porcellanae on its porcelain crab host, Petrolisthes cabrilloi, at four southern California intertidal sites separated by only a few km. The prevalence of rhizocephalan externae varied significantly among sites in 2008-2009, with the southernmost site, White Point, consistently showing higher prevalence than the others. Externa prevalence was a good proxy of estimated true prevalence, i.e., the prevalence of rhizocephalans as a whole, not just those that had formed externae. We examined several hypotheses that might explain the observed spatial variation in prevalence. Host susceptibility to infection (indicated by the proxy of damage to host limbs, some of which are used to remove parasite infective stages), did not differ among sites. At all sites, prevalence was slightly higher in female crabs than in males, and the sex ratio at White Point was slightly female-biased while that at the other sites was mal...